Electrode configurations for semiconductor devices

ABSTRACT

A III-N semiconductor device can include an electrode-defining layer having a thickness on a surface of a III-N material structure. The electrode-defining layer has a recess with a sidewall, the sidewall comprising a plurality of steps. A portion of the recess distal from the III-N material structure has a first width, and a portion of the recess proximal to the III-N material structure has a second width, the first width being larger than the second width. An electrode is in the recess, the electrode including an extending portion over the sidewall of the recess. A portion of the electrode-defining layer is between the extending portion and the III-N material structure. The sidewall forms an effective angle of about 40 degrees or less relative to the surface of the III-N material structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 13/040,940, filed Mar. 4, 2011, the disclosure of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

This invention relates to semiconductor electronic devices, specificallydevices with electrodes connected to field plates.

BACKGROUND

To date, modern power semiconductor diodes such as high-voltage P-I-Ndiodes, as well as power transistors such as power MOSFETs and InsulatedGate Bipolar Transistors (IGBT), have been typically fabricated withsilicon (Si) semiconductor materials. More recently, silicon carbide(SiC) power devices have been researched due to their superiorproperties. III-Nitride (III-N) semiconductor devices are now emergingas an attractive candidate to carry large currents and support highvoltages, and provide very low on resistance, high voltage deviceoperation, and fast switching times. As used herein, the terms III-N orIII-Nitride materials, layers, devices, etc., refer to a material ordevice comprised of a compound semiconductor material according to thestoichiometric formula Al_(x)In_(y)Ga_(z)N, where x+y+z is about 1.

Examples of III-N high electron mobility transistors (HEMTs) and III-Ndiodes, respectively, of the prior art are shown in FIGS. 1 and 2. TheIII-N HEMT of FIG. 1 includes a substrate 10, a III-N channel layer 11,such as a layer of GaN, atop the substrate, and a III-N barrier layer12, such as a layer of Al_(x)Ga_(1-x)N, atop the channel layer. Atwo-dimensional electron gas (2DEG) channel 19 is induced in the channellayer 11 near the interface between the channel layer 11 and the barrierlayer 12. Source and drain contacts 14 and 15, respectively, form ohmiccontacts to the 2DEG channel. Gate contact 16 modulates the portion ofthe 2DEG in the gate region, i.e., directly beneath gate contact 16. TheIII-N diode of FIG. 2 includes similar III-N material layers to those ofthe III-N HEMT of FIG. 1. However, the III-N diode of FIG. 2 onlyincludes two contacts, an anode contact 27 and a cathode contact 28. Theanode contact 27 is formed on the III-N barrier layer 12, and thecathode contact 28 is a single contact which contacts the 2DEG 19. Theanode contact 27 is a Schottky contact, and the single cathode contact28 is an ohmic contact. In FIG. 2, while there appears to be two cathodecontacts, the two contacts are in fact electrically connected so as toform a single cathode contact 28.

Field plates are commonly used in III-N devices to shape the electricfield in the high-field region of the device in such a way that reducesthe peak electric field and increases the device breakdown voltage,thereby allowing for higher voltage operation. An example of a fieldplated III-N HEMT of the prior art is shown in FIG. 3. In addition tothe layers included in the device of FIG. 1, the device in FIG. 3includes a field plate 18 which is connected to gate 16, and aninsulator layer 13, such as a layer of SiN, is between the field plateand the III-N barrier layer 12. Field plate 18 can include or be formedof the same material as gate 16. Insulator layer 13 can act as a surfacepassivation layer, preventing or suppressing voltage fluctuations at thesurface of the III-N material adjacent to insulator layer 13.

Slant field plates have been shown to be particularly effective inreducing the peak electric field and increasing the breakdown voltage inIII-N devices. A prior art III-N device similar to that of FIG. 3, butwith a slant field plate 24, is shown in FIG. 4. In this device, gate 16and slant field plate 24 are formed of a single electrode 29. Insulatorlayer 23, which can be SiN, is an electrode-defining layer that containsa recess which defines at least in part the shape of electrode 29.Electrode-defining layer 23 can also act as a surface passivation layer,preventing or suppressing voltage fluctuations at the surface of theIII-N material adjacent to electrode-defining layer 23. The gate 16 andslant field plate 24 in this device can be formed by first depositingelectrode-defining layer 23 over the entire surface of III-N barrierlayer 12, then etching a recess through the electrode-defining layer 23in the region containing gate 16, the recess including a slantedsidewall 25, and finally depositing electrode 29 at least in the recessand over the slanted sidewall 25. Similar slant field plate structurescan be formed in III-N diodes. For example, a III-N diode similar tothat of FIG. 2 can also include a slant field plate connected to theanode contact 27.

Slant field plates, such as field plate 24 in FIG. 4, tend to spread theelectric fields in the device over a larger volume as compared toconventional field plates, such as field plate 18 in FIG. 3, which donot include a slanted portion. Hence, slant field plates tend to be moreeffective at reducing the peak electric field in the underlying device,thereby allowing for larger operating and breakdown voltages.

While slant field plates are desirable for many applications, they canbe difficult to fabricate reproducibly. Field plate structures that canprovide adequate suppression of peak electric fields and can befabricated reproducibly are therefore desirable.

SUMMARY

In one aspect, a III-N semiconductor device is described that includesan electrode-defining layer having a thickness on a surface of a III-Nmaterial structure. The electrode-defining layer has a recess with asidewall, the sidewall comprising a plurality of steps. A portion of therecess distal from the III-N material structure has a first width, and aportion of the recess proximal to the III-N material structure has asecond width, the first width being larger than the second width. Anelectrode is in the recess, the electrode including an extending portionover the sidewall of the recess. A portion of the electrode-defininglayer is between the extending portion and the III-N material structure.The sidewall forms an effective angle of about 40 degrees or lessrelative to the surface of the III-N material structure.

In another aspect, a III-N semiconductor device is described thatincludes an electrode-defining layer having a thickness on a surface ofa III-N material structure. The electrode-defining layer has a recesswith a sidewall, the sidewall comprising a plurality of steps. A portionof the recess distal from the III-N material structure has a firstwidth, and a portion of the recess proximal to the III-N materialstructure has a second width, the first width being larger than thesecond width. An electrode is in the recess, the electrode including anextending portion over the sidewall of the recess. A portion of theelectrode-defining layer is between the extending portion and the III-Nmaterial structure. At least one of the steps in the sidewall has afirst surface that is substantially parallel to the surface of the III-Nmaterial structure and a second surface that is slanted, the secondsurface forming an angle of between 5 and 85 degrees with the surface ofthe III-N material structure.

Devices described herein may include one or more of the followingfeatures. The III-N material structure can include a first III-Nmaterial layer, a second III-N material layer, and a 2DEG channelinduced in the first III-N material layer adjacent to the second III-Nmaterial layer as a result of a compositional difference between thefirst III-N material layer and the second III-N material layer. Thefirst III-N material layer can include GaN. The second III-N materiallayer can include AlGaN or AlInGaN. A third III-N material layer can beincluded between the first III-N material layer and the second III-Nmaterial layer. The third III-N material layer can include AlN. Thefirst III-N material layer and the second III-N material layer can begroup III-face or [0 0 0 1] oriented or group-III terminated semipolarlayers, and the second III-N material layer can be between the firstIII-N material layer and the electrode-defining layer. The first III-Nmaterial layer and the second III-N material layer can be N-face or [0 00 1bar] oriented or nitrogen-terminated semipolar layers, and the firstIII-N material layer can be between the second III-N material layer andthe electrode-defining layer.

The recess can extend through the entire thickness of theelectrode-defining layer, or into the III-N material structure, orthrough the 2DEG channel. The recess can extend at least 30 nanometersinto the III-N material structure. The recess can extend partiallythrough the thickness of the electrode-defining layer. Theelectrode-defining layer can have a composition that is substantiallyuniform throughout. The electrode-defining layer can include SiN_(x). Athickness of the electrode-defining layer can be between about 0.1microns and 5 microns.

A dielectric passivation layer can be included between the III-Nmaterial structure and the electrode-defining layer, the dielectricpassivation layer directly contacting a surface of the III-N materialadjacent to the electrode. The dielectric passivation layer can includeSiN_(x). The dielectric passivation layer can be between the electrodeand the III-N material structure, such that the electrode does notdirectly contact the III-N material structure. An additional insulatinglayer can be included between the dielectric passivation layer and theelectrode-defining layer. The additional insulating layer can includeAlN. The additional insulating layer can be less than about 20nanometers thick.

The extending portion of the electrode can function as a field plate.The electrode can be an anode, and the device can be a diode. Theelectrode can be a gate, and the device can be a transistor. The devicecan be an enhancement-mode device, or a depletion-mode device, or ahigh-voltage device. The effective angle can be about 20 degrees orless, and a breakdown voltage of the device can be about 100V or larger.The effective angle can be about 10 degrees or less, and a breakdownvoltage of the device can be about 300V or larger.

At least one of the steps can have a first surface that is substantiallyparallel to the surface of the III-N material structure and a secondsurface that is substantially perpendicular to the surface of the III-Nmaterial structure. At least one of the steps can have a first surfacethat is substantially parallel to the surface of the III-N materialstructure and a second surface that is slanted, the second surfaceforming an angle of between 5 and 85 degrees with the surface of theIII-N material structure. The extending portion can directly contact thesidewall.

In another aspect, a method of forming a III-N device is described thatincludes forming an electrode-defining layer having a thickness on asurface of a III-N material structure, and patterning a masking layerover the electrode-defining layer, the masking layer including anopening having a width. The method also includes etching theelectrode-defining layer to form a recess therein, the recess having asidewall which comprises a plurality of steps. A portion of the recessdistal from the III-N material structure has a first width, and aportion of the recess proximal to the III-N material structure has asecond width, the first width being larger than the second width. Themethod further includes removing the masking layer, and forming anelectrode in the recess, the electrode including an extending portionover the sidewall. A portion of the electrode-defining layer is betweenthe extending portion and the III-N material structure. The etching stepincludes a first procedure and a second procedure, the first procedurecomprising removing a portion of the electrode-defining layer, and thesecond procedure comprising removing a portion of the masking layerwithout entirely removing the masking layer. The second procedure causesan increase in the width of the opening in the masking layer.

Methods described herein can include one or more of the following. Thefirst procedure can be performed a second time after the secondprocedure has been performed. The second procedure can be performed asecond time after the first procedure has been performed a second time.The masking layer can include photoresist, and the photoresist in themasking layer can be redistributed prior to performing the etching step.Redistributing the photoresist can include thermally annealing thephotoresist. Redistributing the photoresist can cause the masking layerto have slanted sidewalls adjacent to the opening. The etching step canresult in the recess extending through the entire thickness of theelectrode-defining layer. The etching step can be a first etching step,and the method can further comprise a second etching step resulting inthe recess further extending into the III-N material structure.

The device can further comprise an additional dielectric layer having athickness between the electrode-defining layer and the III-N materialstructure. The etching step can result in the recess further extendingthrough the entire thickness of the additional dielectric layer. Theelectrode can be an anode, and the III-N device can be a diode. Theelectrode can be a gate, and the III-N device can be a transistor. Theetching step can result in the sidewall forming an effective angle ofabout 40 degrees or less relative to the surface of the III-N materialstructure. The etching step can result in at least one of the steps inthe sidewall having a first surface that is substantially parallel tothe surface of the III-N material structure and a second surface that isslanted, the second surface forming an angle of between 5 and 85 degreeswith the surface of the III-N material structure.

III-N devices which can be fabricated reproducibly, can support highvoltages with low leakage, and at the same time can exhibit lowon-resistance and high breakdown voltage, are described. Methods offorming the devices are also described. The III-N devices describedherein can be transistors or diodes, and can be high-voltage devicessuitable for high voltage applications. The details of one or moreimplementations of the invention are set forth in the accompanyingdrawings and description below. Other features and advantages of theinvention will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a III-N HEMT device of the priorart.

FIG. 2 is a cross-sectional view of a III-N diode of the prior art.

FIGS. 3-4 are cross-sectional views of III-N HEMT devices of the priorart.

FIG. 5 is a cross-sectional view of one implementation of a III-N diode.

FIG. 6 is a plan view of the electrode layout of a III-N diode.

FIG. 7 is a cross-sectional view of another implementation of a III-Ndiode.

FIG. 8 is a cross-sectional view of one implementation of a III-N HEMTdevice.

FIG. 9 is a cross-sectional view of another implementation of a III-Ndiode.

FIG. 10 is a cross-sectional view of another implementation of a III-NHEMT device.

FIGS. 11-20 illustrate a method of forming the III-N diode of FIG. 5.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Devices based on III-N heterostructures are described. An electrode ofthe device is designed such that the device can be fabricatedreproducibly, can support high voltages with low leakage, and at thesame time can exhibit low on-resistance. Methods of forming the devicesare also described. The III-N devices described herein can, for example,be transistors or diodes, and can be high-voltage devices suitable forhigh voltage applications. In such a high-voltage diode, when the diodeis reverse biased, the diode is at least capable of supporting allvoltages less than or equal to the high-voltage in the application inwhich it is used, which for example may be 100V, 300V, 600V, 1200V,1700V, or higher. When the diode is forward biased, it is able toconduct substantial current with a low on-voltage. The maximum allowableon-voltage is the maximum voltage that can be sustained in theapplication in which the diode is used. When a high voltage transistoris biased off (i.e., the voltage on the gate relative to the source isless than the transistor threshold voltage), it is at least capable ofsupporting all source-drain voltages less than or equal to thehigh-voltage in the application in which it is used. When the highvoltage transistor is biased on (i.e., the voltage on the gate relativeto the source is greater than the transistor threshold voltage), it isable to conduct substantial current with a low on-voltage. The maximumallowable on-voltage is the maximum voltage that can be sustained in theapplication in which the transistor is used.

Referring to FIG. 4, for a given thickness of the electrode-defininglayer 23, the horizontal length of the region which the electric fieldis spread over, resulting from inclusion of the slant field plate 24, islargely determined by the angle 26 which the field plate forms with thesurface 28 of the underlying III-N material structure. A smaller angle26 results in a greater spreading of the electric fields, allowing forcorrespondingly larger operating and breakdown voltages of the device.For example, in a III-N device with an electrode-defining layer 23 whichis about 0.85 microns thick, an angle of about 40 degrees or less may berequired for reliable 50V or 100V operation, whereas an angle of about10 degrees or less may be required for reliable 300V or 600V operation.However, it can be difficult to reproducibly fabricate slant fieldplates 24 with such small angles 26. A field plate structure whichallows for comparable device operating and breakdown voltages but whichcan be fabricated reproducibly is necessary for large scalemanufacturing.

As illustrated in FIGS. 5-10, the III-N devices described herein aretransistors and diodes that each include an electrode-defining layer ontop of a III-N material structure. The electrode-defining layer includesa recess, and an electrode is in the recess. The width at the top of therecess is greater than the width at the bottom of the recess. Theelectrode includes an extending portion which is over a portion of theelectrode-defining layer and functions as a field plate. The electrodeis deposited conformally in the recess in the electrode-defining regionwith the extending portion over a sidewall of the recess. Hence, theprofile of the extending portion is at least partially determined by theprofile of the sidewall. The sidewall of the recess underneath theextending portion of the electrode includes a plurality of steps. Thesidewall forms an effective angle relative to the uppermost surface ofthe underlying III-N material structure. The effective angle can besmall enough to allow for high voltage operation of the device, asrequired by the circuit application in which the device is used.

Referring to FIG. 5, a III-N diode includes a substrate 10, a firstIII-N layer 11 on top of the substrate, and a second III-N layer 12 ontop of the first III-N layer. III-N layers 11 and 12 have differentcompositions from one another, the compositions selected such that atwo-dimensional electron gas (2DEG) 19 (illustrated by a dashed line),i.e., a conductive channel, is induced in the first III-N layer 11 nearthe interface between the first and second III-N layers 11 and 12,respectively.

An electrode-defining layer 33 is formed over the second III-N layer 12,the electrode-defining layer 33 including a recess in which anodecontact 39 is subsequently formed, extending through the entirethickness of the electrode-defining layer 33. The electrode-defininglayer is typically between about 0.1 microns and 5 microns thick, suchas about 0.85 microns thick. The electrode-defining layer 33 can have acomposition that is substantially uniform throughout. Theelectrode-defining layer 33 is formed of an insulator, such as SiN_(x).An anode contact 39 formed in the recess contacts the upper surface ofthe second III-N layer 12 in region 41 of the device. The anode contact39 includes an extending portion 34, lying over a portion of theelectrode-defining layer 33, that functions as a field plate. The anodecontact 39 is deposited conformally in the recess in theelectrode-defining layer 33 with the extending portion 34 over asidewall 43 of the recess, the sidewall 43 extending from the portion ofthe electrode-defining layer 33 which is closest to region 41 (i.e.,point 44) all the way to the point 45 at the top of theelectrode-defining layer 33 just beyond where the electrode-defininglayer 33 becomes substantially flat. Hence, the profile of the extendingportion is at least partially determined by the profile of the sidewall43. A single cathode contact 28 is formed which contacts the 2DEG 19 andis in close proximity to at least a portion of anode contact 39. Theanode contact 39 is a Schottky contact, and the single cathode contact28 is an ohmic contact.

As used herein, the term “single cathode contact” refers to either asingle metallic contact which serves as a cathode, or to a plurality ofcontacts serving as cathodes which are electrically connected such thatthe electric potential at each contact is about the same, or is intendedto be the same, during device operation. In the cross-sectional view ofFIG. 5, while there appear to be two cathode contacts, the two contactsare in fact electrically connected so as to form a single cathodecontact 28. This is shown more clearly in FIG. 6, which is a plan viewdiagram of the diode of FIG. 5. As used herein, two or more contacts orother elements are said to be “electrically connected” if they areconnected by a material which is sufficiently conducting to ensure thatthe electric potential at each of the contacts or other elements isabout the same, or is intended to be the same, at all times duringoperation.

FIG. 6 is a plan view (top view) of an electrode configuration that maybe used in the device of FIG. 5, and includes alternating “fingers” ofcathode contact 28 and anode contact 39, connected to cathode and anodecontact pads shown on the top and bottom, respectively, of FIG. 6. Whilethe cross-sectional view of FIG. 5 only illustrates a single anodefinger and two cathode fingers, additional cathode and anode fingers canbe added, as shown in FIG. 6.

Referring back to FIG. 5, the diode may also optionally include apassivation layer 22 which contacts the III-N material surface at leastbetween the anode and cathode contacts 39 and 28, respectively, and anadditional dielectric layer 21 between the passivation layer 22 and theelectrode-defining layer 33. The device may also include additionalIII-N layers (not shown), for example a III-N buffer layer between thefirst III-N layer 11 and the substrate 10, or a III-N layer such as AlNbetween the first III-N layer 11 and the second III-N layer 12.

The diode in FIG. 5 operates as follows. When the voltage at the anodecontact 39 is less than that at the cathode contact 28, such that theSchottky junction between anode contact 39 and III-N layer 12 is reversebiased, the diode is in the OFF state with only a small reverse biascurrent flowing between the anode and cathode. Ideally, the reverse biascurrent is as small as possible. When the voltage at the anode contact39 is greater than that at the cathode contact 28, the Schottky junctionbetween anode contact 39 and III-N layer 12 is forward biased, and thediode is in the ON state. In this state, a substantial electron currentflows from the cathode contact 28 predominantly through the 2DEG 19 andthen through the forward biased Schottky junction into the anode contact39. At least 99% of the total forward bias current flows from the anodeto the cathode through the Schottky barrier and through the 2DEGchannel. A small amount of leakage current can flow through other paths,such as along the surface of the device.

As stated earlier, III-N layers 11 and 12 have different compositionsfrom one another. The compositions are selected such that the secondIII-N layer 12 has a larger bandgap than the first III-N layer 11, whichhelps enable the formation of 2DEG 19. If III-N layers 11 and 12 arecomposed of III-N material oriented in a non-polar or semi-polarorientation, then doping all or part of the second semiconductor layer12 with an n-type impurity may also be required to induce the 2DEG 19.If the III-N layers 11 and 12 are oriented in a polar direction, such asthe [0 0 0 1] (i.e., group III-face) orientation, then 2DEG 19 may beinduced by the polarization fields without the need for any substantialdoping of either of the III-N layers, although the 2DEG sheet chargeconcentration can be increased by doping all or part of the second III-Nlayer 12 with an n-type impurity. Increased 2DEG sheet chargeconcentrations can be beneficial in that they can reduce the diodeon-resistance, but they can also lead to lower reverse breakdownvoltages. Hence the 2DEG sheet charge concentration preferably isoptimized to a suitable value for the application in which the diode isused.

III-N materials can be used for layers 11 and 12, the compositions ofthese layers being chosen such that the requirements for layers 11 and12 are satisfied. As an example, III-N layer 11 can be GaN and III-Nlayer 12 can be AlGaN or AlInGaN, whereas layer 12 can be n-doped or cancontain no significant concentration of doping impurities. In the casethat layer 12 is undoped, the induced 2DEG results from the differencein polarization fields between layers 11 and 12. The III-N materialconfigurations for the diode described above can also be used in a III-NHEMT device, as seen, for example, in FIGS. 1, 3, and 4. Hence, thediodes described herein can be integrated with III-N HEMT devices onto asingle chip, thereby simplifying the fabrication process and reducingcost of circuits that require both diodes and HEMTs.

Substrate 10 can be any suitable substrate upon which III-N layers 11and 12 can be formed, for example silicon carbide (SiC), silicon,sapphire, GaN, AlN, or any other suitable substrate upon which III-Ndevices can be formed. In some implementations, a III-N buffer layer(not shown) such as AlGaN or AlN is included between substrate 10 andsemiconductor layer 11 to minimize material defects in layers 11 and 12.

The diode of FIG. 5 optionally includes a passivation layer 22 and anadditional dielectric layer 21. Passivation layer 22, formed of aninsulating dielectric material, such as SiN_(x), atop the second III-Nlayer 12, maintains effective passivation of the uppermost III-N surfaceof the device. As used herein, a “passivation layer” refers to any layeror combination of layers grown or deposited on a surface of asemiconductor layer in a semiconductor device which can prevent orsuppress voltage fluctuations at the surface during device operation,thereby preventing or suppressing dispersion. For example, a passivationlayer may prevent or suppress the formation of surface/interface statesat the uppermost III-N surface, or it may prevent or suppress theability of surface/interface states to trap charge during deviceoperation. Additional dielectric layer 21, which is optionally includedbetween passivation layer 22 and electrode-defining layer 33, cansimplify device fabrication by serving as an etch stop layer. Additionaldielectric layer 21 can be formed of a material for which an etchchemistry exists that can etch the material of electrode-defining layer33 but will not substantially etch the material of additional dielectriclayer 21. Additionally, because the anode contact 39 is in contact withthe underlying III-N materials, the recess in the electrode-defininglayer 33 also extends entirely through the additional dielectric layer21 and the passivation layer 22. In some implementations, passivationlayer 22 and the additional dielectric layer 21 are omitted, andelectrode-defining layer 33 maintains effective passivation of theuppermost III-N surface of the device.

Dispersion refers to a difference in observed current-voltage (1-V)characteristics when the device is operated under RF or switchingconditions, as compared to when the device is operated under DCconditions. In III-N devices, effects such as dispersion are oftencaused by voltage fluctuations at uppermost III-N surfaces, the resultof charging of the surface states during device operation. Accordingly,a passivation layer such as layer 22 prevents or suppresses dispersionby preventing or suppressing voltage fluctuations at the uppermost III-Nsurface.

Referring to the diode of FIG. 5, when passivation layer 22 is included,electrode-defining layer 33 in combination with passivation layer 22maintains effective passivation of the uppermost III-N surface of thedevice. When an additional dielectric layer 21, such as AlN is includedbetween the passivation layer 22 and electrode-defining layer 33, theadditional dielectric layer 21 may need to be made thin enough, such asthinner than about 20 nm, thinner than about 10 nm, or thinner thanabout 5 nm, to ensure that effective passivation of the uppermost III-Nsurface is still maintained. Too thick an additional dielectric layer21, such as greater than about 20 nm, can degrade the passivationeffects of layers 22 and 33.

The portion 35 of anode contact 39 that is formed upon the surface ofIII-N layer 12 forms a Schottky contact to layer 12. Cathode contact 28contacts the 2DEG 19 in ohmic region 49, forming a substantially ohmiccontact. Cathode contact 28 can contact the 2DEG 19 in a number of ways.For example, a metal or combination of metals can be deposited in ohmiccontact region 49 upon the surface of layer 12, followed by a thermalanneal which results in the deposited metal forming a metallic alloywith the underlying semiconductor material. Other methods by which the2DEG can be contacted include, but are not limited to, ion implantationof n-type dopants into ohmic region 49, followed by a metal depositionatop this region, or by etching away the material in ohmic contactregion 49 and regrowing n-type material, followed by a metal depositionatop this region. Anode contact 39 and cathode contact 28 may be anyarbitrary shape, although the shape is ideally optimized to minimize thedevice area required for a given forward current.

The sidewall 43 (and hence also the extending portion 34 of the anodecontact 39) includes a plurality of steps 46. FIG. 5 illustrates thecase where three steps 46 are included. The horizontal width of theuppermost step is defined to be the average width of the other steps inthe structure. Hence, the end of the sidewall 43 closest to the cathodecontact 28 (i.e., the position of point 45) is defined to be at the endof the uppermost step. The sidewall 43 has an effective slope, which isequal to the slope of the dashed line 47 which passes through point 44to point 45. As such, the sidewall 43 forms an effective angle 36 withthe uppermost surface of the underlying III-N material structure.

For a given thickness of the electrode-defining layer 33, a smallereffective angle 36 tends to correspond to a smaller peak electric fieldin the underlying device. Hence, a smaller effective angle 36 tends toresult in a device with a larger breakdown voltage and improvedreliability at higher operating voltages. For example, a device designedto operate at a reverse bias of about 50V or 100V may require aneffective angle 36 which is about 40 degrees or smaller. A devicedesigned to operate at a reverse bias of about 200V may require aneffective angle 36 which is about 20 degrees or smaller, and a devicedesigned to operate at a reverse bias of about 300V or 600V may requirean effective angle 36 which is about 10 degrees or smaller. While slantfield plate structures with angles which are about 40 degrees or smallertend to be difficult to manufacture reproducibly, especially as theelectrode-defining layer 33 is made thicker, stepped field platestructures, such as those shown in FIG. 5, can be much more easilymanufactured reproducibly with effective angles which are about 40degrees or smaller.

As seen in FIG. 5, each of the steps 46 includes two surfaces (althoughthe steps could each include additional surfaces). A first surface ofthe step 46 is substantially parallel to the uppermost surface of theIII-N material structure, while a second surface is at an angle relativeto the uppermost surface of the III-N material structure. The secondsurface of the step 46 can be substantially perpendicular to theuppermost surface of the III-N material structure, or the second surfacecan be slanted, for example forming an angle between about 5 degrees and85 degrees with the uppermost surface of the III-N material structure.While it may be simpler to manufacture a step 46 where the secondsurface is substantially perpendicular to the uppermost surface of theIII-N material structure, making the second surface slanted may bepreferable, as this can further reduce the peak electric field in theunderlying device, as well as allowing for a smaller effective angle 36of the sidewall 43 relative to the uppermost surface of the III-Nmaterial structure.

Another implementation is shown in FIG. 7. The diode in FIG. 7 issimilar to that of FIG. 5, but the recess in the electrode-defininglayer 33 further extends into the III-N materials. As shown, the recesscan extend at least through the 2DEG 19, such that the anode contact 39is in direct contact with the first III-N layer 11 at the bottom of therecess. Furthermore, it has been found that the depth of the recesscontaining anode contact 39 in FIG. 7 (that is, its depth below the 2DEG19) can control or vary shifts in the forward operating voltage V_(on),and correspondingly the reverse bias current I_(reverse), of the device.Changing the depth of the aperture modifies the electric field profilein the III-N materials near the portion of 2DEG 19 modulated by thevoltage on the anode contact 39. A deeper recess reduces the peakelectric field in the region near 2DEG 19 in much the same way as aconventional field plate, thereby leading to devices with higher forwardoperating voltages V_(on), lower reverse bias currents I_(reverse),and/or higher reverse breakdown voltages.

A III-N HEMT transistor which makes use of the stepped field platestructure of the diodes shown in FIGS. 5 and 7 to allow for high voltageoperation as well as simplified and reproducible manufacturingprocedures is shown in FIG. 8. As with the diodes of FIGS. 5 and 7, theIII-N HEMT of FIG. 8 includes a substrate 10, a first III-N layer 11 ontop of the substrate, and a second III-N layer 12 on top of the firstIII-N layer. III-N layers 11 and 12 have different compositions from oneanother, the compositions selected such that a two-dimensional electrongas (2DEG) 19 (illustrated by a dashed line), i.e., a conductivechannel, is induced in the first III-N layer 11 near the interfacebetween the first and second III-N layers 11 and 12, respectively. Anelectrode-defining layer 33 is formed over the second III-N layer, theelectrode-defining layer 33 including a recess which extends through theentire thickness of the electrode-defining layer 33. Theelectrode-defining layer 33 is typically between about 0.1 microns and 5microns thick, such as about 0.85 microns thick. The electrode-defininglayer 33 can have a composition that is substantially uniformthroughout. The electrode-defining layer 33 is formed of an insulator,such as SiN_(x).

A gate 59 is formed in the recess. The gate 59 includes an active gateportion 61 in gate region 51 of the device, as well as an extendingportion 54 which is over a portion of the electrode-defining layer inthe drain access region 53, the extending portion 54 functioning as afield plate. The gate 59 is deposited conformally in the recess in theelectrode-defining region with the extending portion over a sidewall 43of the recess, the sidewall 43 extending from the portion of theelectrode-defining layer 33 which is closest to region 51 (i.e., point44) all the way to the point 45 at the top of the electrode-defininglayer 33, just beyond where the electrode-defining layer 33 becomessubstantially flat. Hence, the profile of the extending portion is atleast partially determined by the profile of the sidewall 43. Source anddrain contacts 14 and 15, respectively, are on opposite sides of thegate 59 and form ohmic contacts to the 2DEG channel 19. The device mayalso include additional III-N layers (not shown), for example a III-Nbuffer layer between the first III-N layer 11 and the substrate 10, or aIII-N layer such as AlN between the first III-N layer 11 and the secondIII-N layer 12.

The III-N HEMT of FIG. 8 has a gate region 51, source and drain accessregions 52 and 53, respectively, on opposite sides of the gate region,and ohmic regions 56. The source access region 52 is between the sourcecontact 14 and portion 61 of the gate, and the drain access region 53 isbetween the drain contact 15 and portion 61 of the gate. As with thediodes of FIGS. 5 and 7, the III-N HEMT of FIG. 8 can also include apassivation layer 22 which contacts the III-N material surface at leastin the access regions, and an additional dielectric layer 21 between thepassivation layer 22 and the electrode-defining layer 33. However, asshown in FIG. 8, the recess in the electrode-defining layer 33 canextend through the entire thickness of the additional dielectric layer21 but not through the passivation layer 22. Hence, passivation layer 22can be between the III-N materials and portion 61 of the gate 59 in thegate region 51, thereby serving as a gate insulator. A gate insulatorcan help prevent gate leakage currents in the HEMT.

The III-N HEMT of FIG. 8 can be an enhancement-mode (i.e., normally off,with a threshold voltage greater than 0V) or a depletion-mode (i.e.,normally on, with a threshold voltage less than 0V) device. Otherconfigurations for the III-N HEMT of FIG. 8 are also possible. Forexample, in one implementation, the recess in the electrode-defininglayer 33 only extends partially through the thickness of theelectrode-defining layer 33, such that a portion of theelectrode-defining layer 33 is between the III-N materials and portion61 of the gate (not shown). In this case, electrode-defining layer 33can also function as a gate insulator, and it may be possible to omitthe passivation layer 22 and/or the additional dielectric layer 21. Inanother implementation, the recess in the electrode-defining layer 33additionally extends through the entire thickness of the passivationlayer 22, and the gate 59 directly contacts the underlying III-Nmaterial (not shown). In yet another implementation, the recess furtherextends into the III-N materials (not shown), such as through the 2DEG19, as with the diode of FIG. 7. In the case where the recess extendsthrough the 2DEG 19, the HEMT can be an enhancement-mode device.

More implementations of devices with stepped field plate structures areshown in FIGS. 9 and 10. FIG. 9 shows a cross-sectional view of a diodesimilar to the device in FIG. 5, but which is fabricated on III-Nsemiconductor material that is either oriented in the N-polar [0 0 01bar] direction or is a nitrogen-terminated semipolar material. That is,the face of the III-N materials furthest from the substrate is either a[0 0 0 1bar] face or is a nitrogen-terminated semipolar face. The deviceincludes a substrate 200 which is suitable for growth of N-polar orsemipolar III-N materials. Layer 201 is a buffer layer, such as GaN orAlN, which reduces the defect density in the overlying III-N material.In some cases, it is possible to omit layer 201 and grow III-N layer 204directly on the substrate 200. The composition of III-N layers 204 and202 are chosen such that a 2DEG 19 can be induced in layer 202 near theinterface between layers 202 and 204. For example, layer 204 can beAlGaN or AlInGaN, and layer 202 can be GaN. An additional III-N layer(not shown), such as a layer of AlN, can be included between III-Nlayers 204 and 202. Electrode-defining layer 33 is similar to or thesame as that used in the diode of FIG. 5. Anode contact 39, which isformed in the recess in the electrode-defining layer 33, contacts thesurface of III-N layer 202 opposite the substrate 200. A single cathodecontact 28 is formed which contacts the 2DEG 19 and is in closeproximity to at least a portion of anode contact 39. The anode contact39 is a Schottky contact, and the single cathode contact 28 is an ohmiccontact. As in the diode of FIG. 5, a passivation layer 22, such as alayer of SiN_(x), can be included on the uppermost surface of the III-Nmaterial structure, and an additional dielectric layer 21, such as alayer of AlN, can be included between the electrode-defining layer 33and the passivation layer 22. Similarly to the diode of FIG. 7, therecess containing the anode contact 39 can also extend into the III-Nmaterial structure (not shown), for example extending through the 2DEG19, and the anode contact 39 can contact III-N layer 204 at the bottomof the recess.

FIG. 10 shows a cross-sectional view of a III-N HEMT transistor similarto the device of FIG. 8, but which is fabricated on III-N semiconductormaterial that is either oriented in the N-polar [0 0 0 1bar] directionor is a nitrogen-terminated semipolar material. The device includes asubstrate 200 which is suitable for growth of N-polar or semipolar III-Nmaterials. Layer 201 is a buffer layer, such as GaN or AlN, whichreduces the defect density in the overlying III-N material. In somecases, it is possible to omit layer 201 and grow III-N layer 204directly on the substrate 200. The composition of III-N layers 204 and202 are chosen such that a 2DEG 19 can be induced in layer 202 near theinterface between layers 202 and 204. For example, layer 204 can beAlGaN or AlInGaN, and layer 202 can be GaN. An additional III-N layer(not shown), such as a layer of AlN, can be included between III-Nlayers 204 and 202. Electrode-defining layer 33, which again includes arecess, is similar to or the same as that of FIG. 8. A gate 59 is formedin the recess. The gate 59 includes an active gate portion 61 in gateregion 51 of the device, as well as an extending portion 54 which isover a portion of the electrode-defining layer in the drain accessregion 53, the extending portion 54 functioning as a field plate. Thegate 59 is deposited conformally in the recess in the electrode-definingregion with the extending portion over a sidewall 43 of the recess, thesidewall 43 extending from the portion of the electrode-defining layer33 which is closest to region 51 (i.e., point 44) all the way to thepoint 45 at the top of the electrode-defining layer 33, just beyondwhere the electrode-defining layer 33 becomes substantially flat. Hence,the profile of the extending portion is at least partially determined bythe profile of the sidewall 43. Source and drain contacts 14 and 15,respectively, are on opposite sides of the gate 59 and form ohmiccontacts to the 2DEG channel 19.

As in the HEMT of FIG. 8, a passivation layer 22, such as a layer ofSiN_(x), can be included on the uppermost surface of the III-N materialstructure, and an additional dielectric layer 21, such as a layer ofAlN, can be included between the electrode-defining layer 33 and thepassivation layer 22. As shown in FIG. 10, the recess in theelectrode-defining layer 33 can extend through the entire thickness ofthe additional dielectric layer 21 but not through the passivation layer22, such that passivation layer 22 also serves as a gate insulator.

The III-N HEMT of FIG. 10 can be an enhancement-mode (i.e., normallyoff, with a threshold voltage greater than 0V) or a depletion-mode(i.e., normally on, with a threshold voltage less than 0V) device. Otherconfigurations for the III-N HEMT of FIG. 10 are also possible. Forexample, in one implementation, the recess in the electrode-defininglayer 33 only extends partially through the thickness of theelectrode-defining layer 33, such that a portion of theelectrode-defining layer 33 is between the III-N materials and portion61 of the gate (not shown). In this case, electrode-defining layer 33can also function as a gate insulator, and it may be possible to omitthe passivation layer 22 and/or the additional dielectric layer 21. Inanother implementation, the recess in the electrode-defining layer 33additionally extends through the entire thickness of the passivationlayer 22, and the gate 59 directly contacts the underlying III-Nmaterial (not shown). In yet another implementation, the recess furtherextends into the III-N materials (not shown), such as through the 2DEG19, as in the diode of FIG. 7.

A method of forming the device of FIG. 5 is illustrated in FIGS. 11-20.Referring to FIG. 11, III-N material layers 11 and 12 are formed onsubstrate 10, for example by metalorganic chemical vapor deposition(MOCVD) or molecular beam epitaxy (MBE). Passivation layer 22, formedover the III-N material layers 11 and 12, is then deposited by methodssuch as MOCVD or plasma enhanced chemical vapor deposition (PECVD).Next, as seen in FIG. 12, a cathode contact 28 is formed which contactsthe 2DEG 19 induced in the III-N material layers. Cathode contact 28 canbe formed in a number of ways. For example, a metal or combination ofmetals can be deposited, for example by evaporation, sputtering, or CVD,in ohmic contact region 49 upon the surface of layer 12, followed by athermal anneal which results in the deposited metal forming a metallicalloy with the underlying semiconductor material. Alternatively, n-typedopants can be ion implanted into ohmic region 49, followed by a metaldeposition by evaporation, sputtering, or CVD, atop this region. Or thematerial in ohmic contact region 49 can be etched away, n-type materialcan be regrown in this region by MOCVD or MBE, and metal can then bedeposited atop this region.

As seen in FIG. 13, the additional dielectric layer 21 andelectrode-defining layer 33 are then deposited over passivation layer22, for example by PECVD, sputtering, or evaporation. A recess is thenetched through the electrode-defining layer, for example by reactive ionetching RIE or inductively coupled plasma (ICP) etching. The procedurefor forming the recess is illustrated in FIGS. 14-19.

Referring to FIG. 14, a photoresist masking layer 71 is patterned on theelectrode-defining 33 to have an opening 72. Patterning can be performedby standard lithography procedures. The photoresist in the masking layer71 is then redistributed, for example by thermally annealing thestructure, resulting in the photoresist profile shown in FIG. 15. Theanneal is performed at a temperature that does not damage thephotoresist layer 71 or any of the underlying layers. As illustrated inFIG. 15, following the redistribution of the photoresist, thephotoresist masking layer has slanted sidewalls 73. The resultingprofile of the photoresist layer 71 and the sidewalls 73 can becontrolled by varying anneal conditions, such as anneal time, annealtemperature, and the chemistry of the ambient gas in which the anneal isperformed. For example, a longer anneal time or a higher temperature mayresult in a smaller slope in the sidewalls 73.

Referring to FIG. 16, the recess in the electrode-defining layer 33 isthen partially formed by performing a first etch employing an etchchemistry that etches both the photoresist in layer 71 and the materialof the electrode-defining layer 33. For example, if theelectrode-defining layer 33 is SiN_(x), the first etch can be performedby Reactive Ion Etching (RIE) or Inductively Coupled Plasma (ICP)etching using an etch chemistry that includes O₂ and SF₆. In someimplementations, the first etch is a substantially anisotropic etch.

As illustrated in FIG. 17, a second etch is then performed which etchesthe photoresist masking layer 71 without substantially etching theelectrode-defining layer 33, thereby increasing the width of the opening72. For example, if the electrode-defining layer 33 is SiN_(x), thesecond etch can be performed by Reactive Ion Etching (RIE) orInductively Coupled Plasma (ICP) etching using an etch chemistry thatincludes only O₂. In some implementations, the second etch is asubstantially isotropic etch. A third etch is then performed which, likethe first etch, utilizes an etch chemistry that etches both thephotoresist in layer 71 and the material of the electrode-defining layer33, resulting in the profile of FIG. 18. The photoresist etch procedure,followed by the procedure for etching both layers 71 and 33, are thenrepeated multiple times, until the recess extends all the way throughthe electrode-defining layer 33, resulting in the aperture having astepped sidewall. The photoresist masking layer 71 is then removed, forexample by a solvent clean, resulting in the profile shown in FIG. 19.Additional dielectric layer 21 can be formed of a material that is notsubstantially etched by the etch procedure used to etch the recess inthe electrode-defining layer 33.

Referring to FIG. 20, the portion of the additional dielectric layer 21which is adjacent to the recess in electrode-defining layer 33 is thenremoved, for example by performing an etch which etches the material ofthe additional dielectric layer 21 but does not etch the material ofelectrode-defining layer 33 or passivation layer 22. For example, whenlayers 33 and 22 are both SiN_(x), and layer 21 is AlN, the portion oflayer 21 adjacent to the recess in electrode-defining layer 33 can bechemically etched in a base, such as a photoresist developer. Next, aportion of the passivation layer 22 adjacent to the recess is etched,for example by RIE or ICP etching, resulting in the structure of FIG.20. Finally, electrode 39 is deposited conformally in the recess, forexample by evaporation, sputtering, or CVD, and optionally the portionof layers 21 and 33 which are over the cathode contact 18 are removed,such as by chemical wet etching, or by RIE or ICP etching, resulting inthe diode of FIG. 5.

The slanted angle of the sidewalls of each step structure in the recessthrough the electrode-defining layer 33 results from the slantedsidewall of the photoresist masking layer 71, as illustrated in FIG. 15.If a vertical sidewall is desired instead of a slanted sidewall for eachstep structure, then the photoresist redistribution procedure describedabove can be omitted, or altered to change the resulting photoresistprofile.

The devices of FIGS. 7-10 can be formed using slightly modified versionsof the methods described above. For example, the device of FIG. 7 can beformed using the procedures described above, with one additional step.Once the recess extends through the passivation layer 22 to theuppermost surface of the III-N materials, and prior to deposition ofelectrode 39, the structure can be etched using an etch chemistry thatetches III-N materials at a higher etch rate than that of the materialsused for electrode-defining layer 33 and passivation layer 22. Forexample, when electrode-defining layer 33 and passivation layer 22 areboth SiN_(x), a Cl₂ RIE or ICP etch can be performed, resulting in therecess extending into the III-N material structure. The device of FIG. 8can be formed using the procedures described above, with the exceptionsthat source and drain ohmic contacts 14 and 15, respectively, are formedin place of the cathode contact 18, and the step of etching thepassivation layer 22 is omitted. The procedures for forming the devicesof FIGS. 9 and 10 are the same as the procedures for forming those ofFIGS. 5 and 8, respectively, with the exception that the III-N layersformed on the substrate in FIGS. 9 and 10 have a differentcrystallographic orientation as compared to the III-N layers formed onthe substrate in FIGS. 5 and 8.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the techniques and devices describedherein. Features shown in each of the implementations may be usedindependently or in combination with one another. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A III-N semiconductor device, comprising: anelectrode-defining layer having a thickness on a surface of a III-Nmaterial structure, the electrode-defining layer having a recess with asidewall, the sidewall comprising a plurality of steps, wherein aportion of the recess distal from the III-N material structure has afirst width, and a portion of the recess proximal to the III-N materialstructure has a second width, the first width being larger than thesecond width; and an electrode in the recess, the electrode including anextending portion over the sidewall, a portion of the electrode-defininglayer being between the extending portion and the III-N materialstructure; wherein at least one of the steps in the sidewall has a firstsurface that is substantially parallel to the surface of the III-Nmaterial structure and a second surface that is slanted; and thesidewall forms an effective angle of about 40 degrees or less relativeto the surface of the III-N material structure.
 2. The device of claim1, wherein the III-N material structure comprises a first III-N materiallayer, a second III-N material layer, and a 2DEG channel induced in thefirst III-N material layer adjacent to the second III-N material layeras a result of a compositional difference between the first III-Nmaterial layer and the second III-N material layer.
 3. The device ofclaim 2, wherein the first III-N material layer includes GaN.
 4. Thedevice of claim 3, wherein the second III-N material layer includesAlGaN or AlInGaN.
 5. The device of claim 2, further including a thirdIII-N material layer between the first III-N material layer and thesecond III-N material layer.
 6. The device of claim 5, wherein the thirdIII-N material layer comprises AlN.
 7. The device of claim 2, whereinthe first III-N material layer and the second III-N material layer aregroup-III terminated semipolar layers, and the second III-N materiallayer is between the first III-N material layer and theelectrode-defining layer.
 8. The device of claim 2, wherein the firstIII-N material layer and the second III-N material layer are N-face or[0 0 0 1bar] oriented or nitrogen-terminated semipolar layers, and thefirst III-N material layer is between the second III-N material layerand the electrode-defining layer.
 9. The device of claim 2, wherein therecess extends through the entire thickness of the electrode-defininglayer.
 10. The device of claim 9, wherein the recess extends into theIII-N material structure.
 11. The device of claim 10, wherein the recessextends through the 2DEG channel.
 12. The device of claim 10, whereinthe recess extends at least 30 nanometers into the III-N materialstructure.
 13. The device of claim 1, wherein the second surface formsan angle of between 5 and 85 degrees with the surface of the III-Nmaterial structure.
 14. The device of claim 1, wherein the recessextends partially through the thickness of the electrode-defining layer.15. The device of claim 1, wherein the electrode-defining layer has acomposition that is substantially uniform throughout.
 16. The device ofclaim 1, wherein the electrode-defining layer comprises SiN_(x).
 17. Thedevice of claim 1, wherein a thickness of the electrode-defining layeris between about 0.1 microns and 5 microns.
 18. The device of claim 1,further comprising a dielectric passivation layer between the III-Nmaterial structure and the electrode-defining layer, the dielectricpassivation layer directly contacting a surface of the III-N materialadjacent to the electrode.
 19. The device of claim 18, wherein thedielectric passivation layer comprises SiN_(x).
 20. The device of claim18, wherein the dielectric passivation layer is between the electrodeand the III-N material structure, such that the electrode does notdirectly contact the III-N material structure.
 21. The device of claim18, further comprising an additional insulating layer between thedielectric passivation layer and the electrode-defining layer.
 22. Thedevice of claim 21, wherein the additional insulating layer comprisesAlN.
 23. The device of claim 21, wherein the additional insulating layeris less than about 20 nanometers thick.
 24. The device of claim 1,wherein the extending portion of the electrode functions as a fieldplate.
 25. The device of claim 1, wherein the electrode is an anode, andthe device is a diode.
 26. The device of claim 1, wherein the electrodeis a gate, and the device is a transistor.
 27. The device of claim 26,wherein the device is an enhancement-mode device.
 28. The device ofclaim 26, wherein the device is a depletion-mode device.
 29. The deviceof claim 1, wherein the device is a high-voltage device.
 30. The deviceof claim 1, wherein the effective angle is about 20 degrees or less, anda breakdown voltage of the device is about 100V or larger.
 31. Thedevice of claim 1, wherein the effective angle is about 10 degrees orless, and a breakdown voltage of the device is about 300V or larger. 32.The device of claim 1, wherein at least one of the steps has a firstsurface that is substantially parallel to the surface of the III-Nmaterial structure and a second surface that is substantiallyperpendicular to the surface of the III-N material structure.
 33. Thedevice of claim 1, wherein the extending portion directly contacts thesidewall.
 34. A III-N semiconductor device, comprising: anelectrode-defining layer having a thickness on a surface of a III-Nmaterial structure, the electrode-defining layer having a recess with asidewall, the sidewall comprising a plurality of steps, wherein aportion of the recess distal from the III-N material structure has afirst width, and a portion of the recess proximal to the III-N materialstructure has a second width, the first width being larger than thesecond width; and an electrode in the recess, the electrode including anextending portion over the sidewall, a portion of the electrode-defininglayer being between the extending portion and the III-N materialstructure; wherein at least one of the steps in the sidewall has a firstsurface that is substantially parallel to the surface of the III-Nmaterial structure and a second surface that is slanted, the secondsurface forming an angle of between 5 and 85 degrees with the surface ofthe III-N material structure.
 35. The device of claim 34, wherein theIII-N material structure comprises a first III-N material layer and asecond III-N material layer, wherein a 2DEG channel is induced in thefirst III-N material layer adjacent to the second III-N material layeras a result of a compositional difference between the first III-Nmaterial layer and the second III-N material layer.
 36. The device ofclaim 35, wherein the first III-N material layer includes GaN.
 37. Thedevice of claim 36, wherein the second III-N material layer includesAlGaN or AlInGaN.
 38. The device of claim 35, wherein the first III-Nmaterial layer and the second III-N material layer are N-face or [0 0 01bar] oriented or nitrogen-terminated semipolar layers, and the firstIII-N material layer is between the second III-N material layer and theelectrode-defining layer.
 39. The device of claim 35, wherein the recessextends through the entire thickness of the electrode-defining layer.40. The device of claim 39, wherein the recess extends into the III-Nmaterial structure.
 41. The device of claim 34, wherein the recessextends partially through the thickness of the electrode-defining layer.42. The device of claim 34, further comprising a dielectric passivationlayer between the III-N material structure and the electrode-defininglayer, the dielectric passivation layer directly contacting a surface ofthe III-N material adjacent to the electrode.
 43. The device of claim42, wherein the dielectric passivation layer is between the electrodeand the III-N material structure, such that the electrode does notdirectly contact the III-N material structure.
 44. The device of claim43, further comprising an additional insulating layer between thedielectric passivation layer and the electrode-defining layer.
 45. Thedevice of claim 44, wherein the additional insulating layer comprisesAlN.